Yankee dryers have been used for many years to dry wet paper webs during the paper making process. Typically, the wet paper web is delivered to and pressed into engagement with the rotating dryer drum with the outer cylindrical surface of the dryer drum delivering the wet paper web to a dryer hood associated with the dryer drum. The dryer drum is heated internally by steam or the like so that the outer surface thereof is maintained at a high temperature. After entering the dryer hood the wet paper web is also subjected to heat on the outer surface (air side) thereof.
A yankee dryer is typically used to produce tissue creped from the yankee dryer drum after passage through the hood. Tissue on a yankee dryer drum will blister when the surface of the dryer drum transfers heat to the sheet too rapidly. The sheet is ruined and operation is disrupted while the problem is being corrected.
Blistering tends to occur in one spot rather than across the entire web. Yankee dryer coating, felt cleanliness and wear, basis weight and yankee dryer condensate removal all can effect blistering.
The machine operator typically attempts to resolve matters by reducing the yankee dryer drum steam pressure and increasing hood temperature. Air side drying, that is drying induced at the outer web surface, is considerably more expensive than steam side drying, that is drying caused by the heated dryer drum as applied to the inner sheet surface. Thus, the common practice of reducing the yankee steam pressure and increasing hood temperature results is a poor one from the standpoint of economizing. This is aggravated by the fact that the machine operator customarily allows himself a wide safety margin so that the problem does not show up during speed changes or other transient conditions on the machine. On drying limited machines blistering can result in reduced production.
A search of the prior art located the following U.S. Pat. 1,698,818, issued Jan. 15, 1929, 1,830,287, issued Nov. 3, 1931, 3,213,858, issued Oct. 26, 1965 and 4,665,631, issued May 19, 1987.
None of the above-identified patents addresses the problem of blistering of a wet paper web on a yankee dryer drum.
U.S. Pat. No. 3,212,858, however, does disclose a drying drum wherein a slurry of tobacco is deposited on the drum and formed into a self-supporting sheet or film. A forced air supply jet is provided for directing a stream of air toward the slurry web on the dryer drum to keep water vapor pressure down and prevent boiling while a dispersion is still in liquid form. The air impinges on the dispersion immediately after forming on the drum surface to accelerate the drying rate. The patent provides a general suggestion to the effect that the method can be used for the preparation of sheet material from dispersions or slurries other than tobacco.
The forced air jet of U.S. Pat. No. 3,213,858 is not disclosed with particularity; however, it appears that the jet uniformly and continuously directs compressed air along the full length of the dryer drum, that is along the full width of the slurry film. There is no teaching of employing a plurality of air jets which are independently controlled to direct air to selected different incremental width portions of the slurry. Also, the air stream in the patent is for the purpose of cooling the dispersion at the point of contact between the roll applying the dispersion to the drum and the drum. This will not solve the problem of blistering in the zone between the applicator roll and the exhaust hood shown in the patent.
U.S. Pat. No. 1,830,287 discloses a drum for heating paper wherein a mantle is provided over a portion of the drum and air is supplied thereto to assist in the drying of the paper. The air may be pre-heated to about the same temperature as the air leaving the chamber. The device supplies air to the drum by utilizing a manifold. There is no teaching that the apparatus can be utilized for blister control or prevention.
U.S. Pat. No. 1,698,818 is directed to a device for moistening paper by use of steam. A steam box or chest 26 is associated with a source of steam whereby the steam is supplied to an area adjacent to the cylinder of the apparatus for moistening the paper as the paper is conveyed therealong. Again, this approach is not useful for control of blistering on a yankee dryer drum.
U.S. Pat. No. 4,665,631 is directed to a high speed paper machine dryer. Air nozzles are provided adjacent to dryer rolls in order to provide an sealing nozzle at the locations where pressure build-up problems exist. This arrangement is quite different than that disclosed herein and is inapplicable insofar as providing a solution to blistering is concerned.